As a childfree woman, it's a no brainer that friends become far less available when they have kids. It's a no brainer that the nature and frequency of catch-ups will change. (For example, weekly catch-ups at the pub might instead become a cup of team at your friend's house every couple of months). It's also a no brainer that new mums might want to seek out the comfort and support of other new mums during those early years - and no reasonable childfree person would begrudge them that!
However, I don't think that's what the OP is talking about. I get the feeling from her posts that the OP is talking about a situation where the kids might be a bit older and more independent, but her friends still refuse to make any time for her whatsoever; instead, preferring to spend all their spare social time with 'mum friends' on the basis that the OP (and other friends without kids) simply don't fit into their lives anymore.
If that's correct (and please do correct me if I'm wrong, OP!), I totally understand how that would sting! It always hurts to be the one who is 'ditched'. And that goes both ways, for people with and without kids.
As for the "why", who really knows? As you and others have said, there can be a mentality among some women where they only see themselves as "mum", and spending time with old friends who don't have kids somehow challenges that identity?
Lack of time is often a big one, of course. And I get that "it's just easier" to get together with other friends who have kids. But even so, if you value your childfree friends, you'd find a way to make time for them every so often. Even if it's just say, a quick coffee for an hour, every few months, to keep the friendship ticking along.
I think another unspoken reason might be the level of support from the spouse/father of the kid(s). My personal experience has been that it's much easier to keep up friendships where the dad is supportive, and willing to watch the kid(s) once in a while. If the dad is not willing (or is incompetent and can't be trusted / can't cope), it's much harder. Because then my friend either has to decline invitations, or bring the kids along (which makes it harder to have a proper conversation, etc).